$ 4^{-\frac{1}{2}}$
Explanation: $= \left(\dfrac{1}{4}\right)^{\frac{1}{2}}$ Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{1}{4}$ Figure out what goes in the blank: $\Big({\dfrac{1}{2}}\Big)^{2}=\dfrac{1}{4}$ So $4^{-\frac{1}{2}}=\left(\dfrac{1}{4}\right)^{\frac{1}{2}}=\dfrac{1}{2}$